Attention is directed to patent publication U.S. Pat. No. 4,465,430. The present invention represents an improvement to the technology described therein.
In an axial-flow (propeller-type) turbine, water enters and leaves the blades with little or no radial component of motion in passing through the blades. In passing through the propeller blades, however, the water does undergo a rotational displacement. The energy for providing the rotational velocity is, of course, derived from the mechanical energy contained in the water; and ideally there should be no rotational velocity in the water leaving the turbine.
As described in the said U.S. Pat. No. 4,465,430, the incoming water can be fed through a guide-case, which is shaped to impart a rotational (or rather, helical) velocity to the water. Thus, a rotational component is imparted to the overall velocity of the incoming water. It was found that shaping the guide-case so as to impart this extra rotational velocity enabled more of the energy of the incoming water to be converted into electricity.
The maximum possible rate at which energy can be extracted from the incoming water is the product of the pressure and the volumetric flowrate (PV) of the water; providing the guide-case enables more electricity to be generated from a given water PV. The energy for creating the rotational velocity of course can only be derived from the PV of the water in the headrace, and creating the rotational velocity must drain some energy from the water; but the effect, overall, is to enable more of that energy to be fed into the turbine shaft, and to enable the water in the tailrace to be more nearly completely spent.
In the guide-case shown in FIGS. 1 to 6 of U.S. Pat. No. 4,465,430, water enters the turbine blades 32 (using the numerals of that publication) through a turbine chamber 21. The turbine chamber has an annular configuration; its inner diameter is the turbine shaft 33 (or rather, its inner diameter is the sleeve 25 around the turbine shaft), and its outer diameter is the outer casing 24 surrounding the turbine blades 32.
In '430, the headrace 16 containing the incoming water is of a long-by-narrow shape. This headrace is disposed with its long length parallel to the axis of the turbine. The narrow width of the headrace occupies the radial annular width of the annular chamber. The far end wall 18 of the headrace rectangle blends into a helical wall 26. This helical wall completes (almost) one complete turn around the inner sleeve 25.
In '430, the effect is that the far water stream, which enters the annular chamber from the far end of the rectangular headrace, travels axially along the annular chamber and in doing so completes one rotation around the annulus; this water then meets and mixes with the near water stream, which has just entered the annular chamber from the near end of the long-by-narrow headrace.
It will be understood that the near and far waters are not two separate streams: rather, they are the near and far ends of the single stream that is entering the turbine from the headrace. It may be regarded that the headrace stream wraps itself around the turbine shaft, and the near and far ends of that stream blend into each other in order to form the complete annulus of water that enters the annular chamber.
Ideally, the near water, which enters the annular chamber from the near end of the headrace, should perfectly match the far water, which enters from the far end, and which has already completed one revolution of the annular chamber. That is to say, the near water and the far water should match each other, at the point where they intersect, as far as their rotational velocities are concerned.
In '430, as a matter of fact, the problem arises that the near and far waters did not match very well. The present invention is aimed at improving the match. The improvement means that the turbulences and cross-currents arising from the mismatch between the two water-streams are eliminated, or reduced, with a consequent improvement in energy efficiency. It is recognised, in the invention, that the near and far water-streams can be induced to match each other more closely, leading to a desired improvement in efficiency, simply by making a rather modest change in the structure of the guide-case.